1. Field of the Invention
The present invention relates to lubrication systems for earth boring drill bits, and in particular to pressure compensators for equalizing lubricant pressure in the system with respect to bore hole fluid, as well as to the methods by which such systems are filled and installed.
2. Description of Related Art
Earth boring drill bits use various types of lubrication systems, and in the case of rotatable cutter type earth boring drill bits, in particular, it is common for lubrication systems to be used which include a pressure compensator to minimize the pressure differential across a bearing seal used in the drill bit. Typically, such a pressure compensator for a lubrication system of an earth boring drill bit uses a flexible diaphragm which is sealed in a lubricant reservoir so that the lubricant is in contact with one side of the pressure compensator diaphragm, and the well bore fluid is in contact with the opposite side thereof for purposes of equalizing the lubricant pressure with the pressure of the fluid in the well bore. Numerous types of pressure compensators are known, but, generally, they can be classified into one of two main categories, i.e., top vented and bottom vented pressure compensators.
Examples of drill bits having bottom vented pressure compensators can be found, for example, in U.S. Pat. No. 3,476,195, issued to Galle, as well as in U.S. Pat. Nos. 4,055,225 and 4,276,946 of the present inventor. An advantage of using a bottom vented pressure compensator, as shown in these patents, is that well bore fluid and cuttings are able to drain through the vent hole of the pressure compensator as the bit is being removed from the well since the vent hole is located in the lower end of the compensator. On the other hand, a disadvantage of this type of lubrication system is the necessity to seal the pressure compensator in the bottom portion of the lubricant reservoir, in addition to sealing a compensator cap which closes the upper end of the reservoir. Furthermore, bottom vented compensators face the problem that vent holes through which the well hole materials are forced to fill the area below the pressure compensator can become partially or totally blocked with well hole cuttings in a manner preventing proper operation of the compensator which, in turn, can result in damage to the bearing seals. A pressure compensator that is vented to the well bore fluid through the top of the lubricant reservoir is not subject to such difficulties.
Examples of various types of top vented compensators for earth boring drill bits can be found in Garner U.S. Pat. No. 3,917,028, the present inventor's U.S. Pat. No. 3,942,596, and U.S. Pat. No. 4,552,228, issued to Evans, et al. The diaphragm of top vented compensators is generally cup-like in shape, and whether of the folded wall type (U.S. Pat. Nos. 3,942,596 and 3,917,028) or of the elongated radially expansible type (U.S. Pat. No. 4,552,228) is provided with a mounting flange at an open upper end thereof. This flange typically has a greater diameter than that of the lubricant receiving space of the lubricant reservoir so as to be supported upon a shoulder of the reservoir, against which it is pressed by a cap of the compensator through which venting is achieved. Furthermore, in most instances, an inner surface of the open end of the diaphragm seats on a peripheral surface of a reduced diameter inner end of the cap of the compensator for improving the sealing produced as the cap presses the flange of the diaphragm against the reservoir shoulder.
To prevent damage to the diaphragm when the well hole fluid pressure is much greater than that of the lubricant, due to contact between the diaphragm and sharp edges of the lubricant reservoir, conventionally a metal stiffener or protector sleeve or cup is disposed about at least the upper end of the cup-like diaphragm or is molded into the wall of the diaphragm at the upper end thereof. In this way, the diaphragm cannot be damaged by, for example, the sharp edge formed at the junction of the lubricant reservoir and the passage by which the lubricant receiving space of the reservoir communicates with the bearings of the cutters, nor can the upper end of the diaphragm be extruded into that passage or down into the reservoir. However, the necessity for such a diaphragm supporting metal sleeve or cup complicates and increases the cost of the compensator of the lubrication system.
Additionally, besides the problems associated with a high pressure differential of the well hole fluid relative to the lubricant pressure, the need exists to avoid damage to the bearing seals as a result of an excessive lubricant pressure generated during raising or lowering of the drill bit, or occurring as a result of lubricant vaporization due to the heat and pressure imposed during operation of the drill bit. One method to eliminate the buildup of internal pressure is to use a pressure relief valve as is shown in U.S. Pat. No. 3,942,596. However, a relief valve, besides increasing costs, is an additional part of the lubrication system that must be sealed to prevent entrance of the well bore fluid into the lubricant side of the system. That is, failure of the relief valve or the seal around the relief valve can cause the lubrication system to fail. Furthermore, relief valves suffer the disadvantage that lubricant can leak out of the system when the valve opens.
Another method of reducing lubrication pressure buildup is to allow space in the reservoir for lubricant expansion. This can be done by removing a given amount of the lubricant after filling of the drill bit with lubricant. Alternatively, as in the case of Garner U.S. Pat. No. 3,917,028, when lubricant is to be introduced into the reservoir, a temporary cap is held in place in the open end of the reservoir instead of the cap of the compensator. This temporary cap is in the form of a closure plug having an inner end surface which extends a distance inwardly relative to the cup-like diaphragm that is significantly closer to the bottom of the lubricant reservoir than the bottom of the permanent cap does. With the plug and diaphragm installed, lubricant is pumped into the reservoir through a port near the bottom of the reservoir until the reservoir is filled to an extent pressing the diaphragm against the underside of the temporary cap. Thus, when the temporary cap is replaced by the compensator cap, a lubricant-free space will be provided between the upper side of the flexible diaphragm and the bottom portions of the cap of the compensator. This lubricant-free space can serve as an expansion space which will at least partially, if not entirely, relieve vaporized lubricant pressure buildup in the lubrication system. Thus, it is possible to close the lubricant filling port of the Garner reference by either a high pressure relief valve or a simple filler plug. On the other hand, by using a plug-like temporary filling cap that is deeper than the permanent compensator cap and then filling the reservoir with the diaphragm in place, an undesirable underfilling of the reservoir results.
All of the lubrication systems described above are relatively complex. These prior art systems utilize pressure compensators that have numerous areas that require very special attention to assure that sealing will be of the quality that will prevent lubricant loss and prevent leakage of bore hole fluid into the rock bit lubrication system. As such, even though such systems have been successful, a lubrication system having a pressure compensator of a simpler and more reliable design is needed.